replace get_block with get_block_header_state

[UMU618]

Improve #581

[mmcs85]

Hey there guys I want to say this improvement is great! I though about using get_block_header_state before but didn't know how to extract the prefix.

There is a constraint though when using it.
get_block_header_state does not return blocks before the LIB.
(nodeos prob fetching from reversible blocks database)

{"code":400,"message":"Unknown Block","error":{"code":3100002,"name":"unknown_block_exception","what":"Unknown block","details":[{"message":"Could not find reversible block: 44517609","file":"chain_plugin.cpp","line_number":1875,"method":"get_block_header_state"}]}}

Meaning that this will fail for 2 cases:
1 - local testnets with 1 producer "eosio" that the LIB distance between the Head is 1 block. The reason is prob that the fetch is slower than the info is on nodeos tables.
2 - blocksBehind > 300 blocks

I have implemented in eos-sharp a fallback to get_block for those cases here:
https://github.com/GetScatter/eos-sharp/blob/master/EosSharp/EosSharp.Core/EosBase.cs#L409
Keep it up!

[tbfleming]

Dang it. @c0d3ster we'll have to add a fallback or revert the change.